Vacuum-type circuit interrupters having an axial magnetic field produced by condensing shield coils

ABSTRACT

A vacuum-type circuit interrupter is provided having an axial magnetic field provided therein through the use of currents transmitted to the surrounding shield structure, the latter being electrically connected by means of a field coil to one of the separable contacts. In one embodiment, the surrounding condensing shield is connected by an internal coil strap to one of the separable contacts. In another embodiment, the coil, providing the axial magnetic field, is disposed externally of the evacuated envelope, and is connected by a connection extending through the envelope to the condensing shield, which may be electrically isolated internally from the electrodes or contacts. In another embodiment, the axial magnetic field-producing coil is disposed externally of the envelope, but has a connection to an end plate, which, additionally, is electrically connected to the internal condensing shield. Another embodiment has the condensing shield provided in the form of a helical conducting strap. In another embodiment, a split condensing shield is connected by strapcoil connecters to both of the separable contacts of the vacuum interrupter for creating an axial magnetic field. In another embodiment, the &#39;&#39;&#39;&#39;floating&#39;&#39;&#39;&#39; shield has a generally spiral configuration adjacent to the arcing gap. The &#39;&#39;&#39;&#39;floating&#39;&#39;&#39;&#39; shield will draw net electron current on the cathode side of the shield, and a compensating ion current on the anode side. The resulting circulating current will cause an axial magnetic field to be set up interiorly of the vacuum interrupter. In yet another embodiment of the invention the condensing shield is connected by a field coil to one of the fixed electrodes of a fixed-gap vacuum device to assist in extinguishing the arc established between the two fixed electrodes.

United States Patent [191 Emmerich et al.

[ June 10, 1975 [75] Inventors: Werner S. Emmerich; Clive W.

Kimblin; Richard L. Hundstad, all of Pittsburgh, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Sept. 14, 1973 [21] Appl. No.: 397,564

[52] US. Cl. 200/144 B; 200/147 R [51] Int. Cl. H01h 33/66 [58] Field of Search ZOO/144 B, 147 R [56] References Cited UNITED STATES PATENTS 3,702,91 l 1 H1972 Schonhuber ZOO/144 B Primary Examiner-Robert S. Macon Attorney, Agent, or Firm-W. R. Crout [5 7] ABSTRACT A vacuum-type circuit interrupter is provided having an axial magnetic field provided therein through the use of currents transmitted to the surrounding shield structure, the latter being electrically connected by means of a field coil to one of the separable contacts.

In one embodiment, the surrounding condensing shield is connected by an internal coil strap to one of the separable contacts. In another embodiment, the coil, providing the axial magnetic field, is disposed externally of the evacuated envelope, and is connected by a connection extending through the envelope to the condensing shield, which may be electrically isolated internally from the electrodes or contacts. In another embodiment, the axial magnetic field-producing coil is disposed externally of the envelope, but has a connection to an end plate, which, additionally, is electrically connected to the internal condensing shield.

Another embodiment has the condensing shield provided in the form of a helical conducting strap. In another embodiment, a split condensing shield is connected by strapcoil connecters to both of the separable contacts of the vacuum interrupter for creating an axial magnetic field. In another embodiment, the floating shield has a generally spiral configuration adjacent to the arcing gap. The floating shield will draw net electron current on the cathode side of the shield, and a compensating ion current on the anode side. The resulting circulating current will cause an axial magnetic field to be set up interiorly of the vacuum interrupter.

In yet another embodiment of the invention the condensing shield is connected by a field coil to one of the fixed electrodes of a fixed-gap vacuum device to assist in extinguishing the are established between the two fixed electrodes.

18 Claims, 29 Drawing Figures PKYEQH'EMUH 10 ms .i 8 89 179 SHEET PULSE SOURCE SHEET FIG.4

SHEET ARC VOLTAGE, V N 01 ()1 O AERO AXIAL MAGNETIC FIELD 'SATURATED ION CURRENT |QQ ///\COIL"LOADLINE" 'l I l l O o 200 400 600 800 MAGNETIC FIELD GAUSS SHEET I PATFEJ EMN 1 0 ms FIG.8YA

[90M CURRENT i ANODE ELECTRODE III" PLATE (D) 1 @-AMMETER ,-METAL (n 'tl WARC PLASMA \4! Fr I (ii) ANODE ,ELECTRQDE FIGBB VARC PLASMA I INCREASING NUMBER OF TURNS :00 IODOEIP .rzwmmDo NCATHOVDE ELECTRODE vml B v S m w M T Um law NR D U N 7 M 5 F l MO F E m m H m w M 5 5 V A m & O .1 N S F C l R T D E Lw N E G G H I! M S F FIXED ELECTRODE MULTI -TURN l9 FIELD COIL FIG.I2A

MOVABLE ELECTRODE- ICATHODE) CLOSING SWITCH] \CAPACITOR BANK TEST INTERRUPTER (FIG. I2A) I SHIELD CURRENT MEASURING SHUNT \ARC CURRENT MEASURING SHUNT ARC VCEACE- FIG.|2

\ ARC VOLTAGE MEASURED VIA VOLTAGE DIVIDE RS Arc Power, MW P o 9 b O\ 00 SHEET 8,

Peak Arc Voltage, V

I l I I l l l Time after Current Zero, ms

FIG.2O

ak Arc Current,.kA I l 200 l l l Arc Voltage (shield connected directly to cathodel o o o v l Arc Voltage Arc Voltage l12lurns'shieldto cathode) l6turns shield to cathode) s A 1o 1s Arc Current, 'kA lr. m. s.) FIG. II:

B 8 8 Arc Voltage, V

Arc Power, MW

SHEET I 9 a 2.0 VOLTAGE 3 a: E a; F lG.l5 E U 2 5 Time after Current Zero, ms

5 VOLTAGE -30 E POWERA E Time after Current Zero, ms

FIG.I4

ikTBlTFMus: 10 ms Peak Arc Current, kA

Shield Direct to Cathode Shield to Cathode via 6 Turns Shield to qq thode via 12 Turns Arc Current, kA (r. msg) FIGJB FIG.I7"

lkA

Emmmau 29 TI E TIME kzmmmau 29 6"TURNS IN FIELD COIL O-TURNS IN FIELD COIL IFTIME FZmmmDO 29 IZ-TURNS IN FIELD COIL T AYETJTEDJUA 10 m5 8.883079 SiEET T2 NET ION CURRENT APPROX. |o% OF"I" ACTS gAsED 6 ATIVE R1El /XHe/E CATHODE CATHODE PLASMA NET ION CURRENT NET ION CURRENT H623 ARC VOLTA REDUCED l9 BIASED POSITIVE RELATIVE ACTS To ARC 'flfg PLASMA ANODE ELEQJETTRON ELE C T RDN CURRENT CATHODE I CURRENT ARC VOL AGE REDUCED PMENTEDM 10 I975 3,889,079

' SHEET 1 3 CAT HODE FIG.25

AXIAL MAGNETIC FIELD /ANODE DIRECT CURRENT ARC (I,OOO)

[CATHODE FIG.26

NO AXIAL. MAGNETIC FIELD DIRECT CURRENT ARC 0,000)

1 VACUUM-TYPE CIRCUIT, INTERRUPTERS HAVING AN AXIAL MAGNETIC FIELD PRODUCED BY CONDENSING SHIELD COILS CROSS-REFERENCES TO RELATED APPLICATIONS Applicants are not aware of any related patent applications pertinent to the present invention.

BACKGROUND OF THE INVENTION It has been known by those skilled in the art that certain benefits are to be achieved by utilizing an axial magneticfield interiorly of a vacuum bottle, or vacuum-type circuit interrupter. For example, reference may be made to U.S. Pat.: Lee 3,321,599; British Pat. No. 1,258,015; French Pat. No. 1,415,441; Polinko, Jr. et al. No. 3,345,484; Lucek et a1. U.S. Pat. No. 3,263,162. Generally, the coils have been in series with the separable contacts of the vacuum interrupter.

Several investigators have disclosed that axial magnetic fields are effective in increasing the current interruption ratingof vacuum interrupters. This axial magnetic field is applied parallel to the axes of the electrodes, and the field-producing coils are always placed in series with the current through the electrodes. In patents by Lee U.S. Pat. Nos. 3,321,599 and 3,372,259, 3,372,258 by Porter, the axial magnetic field is produced by a solenoidal coil wound external to the interrupter. In a third U.S. Pat. No. 3,158,722 by Porter, the electrode stems are constructed from materials of widely different electrical conductivities, and an axial magnetic field is produced from these stem coils.

The axial magnetic field is effective in increasing the current interruption ability of a vacuum interrupter for two reasons. First, the arc voltage is reduced during the critical half cycle of arcing prior to the natural current zero. This arc voltage reduction minimizes the energy released within the interrupter, and consequently minimizes the temperature rise of the internal interrupter components. In particular, recent experiments indicate that axial fields produce a significant increase in the threshold current associated with anodespot formation. Thus, in low-current dc experiments, where a small diameter anode is drawn to long electrode spacings from the cathode, the threshold current for anodespot formation in the absence of an axial magnetic field is approximately 500 amperes. When the experiment is repeated, with the axial field of approximately 300 gauss, however, the threshold current for anode-spot formation is raised to approximately 1500 amperes. Recent literature reveals that low-current anode-spot experiments are relevant to high-current phenomena in vacuum interrupters, and we therefore feel confident that axial magnetic fields will delay the onset of anodespot formation in a practical alternating-current interrupter.

A second benefit derives from the confining effect of an axial magnetic field. This field reduces the plasma contact with the vapor-condensation shield, and therefore reduces the probability of the are striking to the condensing shield. This is a particularly important effect since many cost-reduced vacuum interrupters are being manufactured with the vapor-condensation shield connected directly to one of the. electrodes. We have shown that the rate of rise ofdielectric strength is only decreased slightly in these integral shield designs, but problems may be encountered at high currents (approximately 20 kA) due to the tendency for the arc to strike over to the'shield. The strike-over tendency will be small when the shield is connected to the anode electrode during arcing. In fact, this is a beneficial connection since the shield acts as an auxiliary anode and reduces the arc voltage. However, the shield will be biased negative relative to the arc plasma during those arcing half-cycles in which the shield is at cathode potential. This connection does not reduce the arc voltage. Rather, under these circumstances, the arc plasma may initiate cathode spots on the shield with resulting catastrophic failure of the interrupter. We can conclude that axial magnetic fields will definitely prove of value in cost-reduced interrupters, and the field will prove of particular value when the shield is at cathode potential during the arcing half-cycle.

SUMMARY OF THE INVENTION In the present invention, we are now disclosing a method for applying axial magnetic fields to highcurrent vacuum arcs without the use of a seriesconnected coil. In our system, the coil is essentially connected in parallel with the arc, and only conducts current during the arcing half-cycles. The coil iseffectively short-circuited during normal alternating-current operation with the electrodes or contacts closed, and thus power losses, due to eddy-current'heating of the interrupter end plates, are non-existent. Our embodiments rely on the fact that currents flow from the arc plasma to the shield when the latter is connected to either anode or cathode potential, this connection being made through a coil. When the shield is connected to anode potential, the shield draws electron current from the arc plasma as it is this appreciably large electron current flowing through the coils that creates the axial magnetic field. This axial magnetic field lowers the arc voltage, and also reduces, to some extent, the plasma contact with the shield. More importantly, we have discovered that large ion currents also flow from the plasma to the shield when the shield is connected to cathode potential. This ion current can be of the order of 10% of the arc current. Thus, for those AC current cycles when the shield is connected via its coil to cathode potential, appreciable axial magnetic fields are also created.

In accordance with preferred embodiments of the present invention, there are provided various condensing-shield configurations in which the ion or electron current, collected by the condensing shields, is utilized in an advantageous manner to result in an axial mag netic field provided interiorly of the vacuum envelope to result thereby in a confinement of the are without serious anode-spot formation and a rapid extinction thereof following low arc voltage.

In one embodiment of the invention there is provided an internal coil, which is electrically connected between the stationaryy contact, or electrode and the internally-disposed condensing shield. In another embodiment of the invention, an externally-located coil is provided being connected electrically between the stationary-contact stem and, by means of a probe connection, to the internally disposed condensing shield.

In still another embodiment of the invention an externally-located coil is electrically connected between the stationary contact stemand an end plate of the interrupter with, preferably, the internally-disposed condensing shield affixed to said end plate, so that the externally located coilis connected between the stationary contact and the internally-located condensing shield.

Yet another embodiment of the present invention provides a generally spiral coil disposed internally of the envelope, and supported from the end plate associated with the stationary-contact end of the vacuum circuit-interrupter. This coil constitutes the internallylocated condensing shield for the interrupter. v

In yet another embodiment of the invention the internally-located condensing shield is of split construction,

one half of the split condensing shield being electrically connected by a coil to the stationary contact of the interrupter. The other half of the split condensing shield is electrically connected by a second coil to the movable contact of the interrupter. The two coils act like a Helmholz coil in a cumulative fashion to provide an internal axial magnetic field.

In yet another embodiment of the present invention there is provided an isolated shield in which the shield is electrically floating, and is not connected to either electrode. This shield has a generally spiral configuration. It is anticipated that parts of the shield adjacent to the cathode electrode will collect predominantly electronicurrent. Parts of this shield adjacent to the anode electrode will collect predominantly ion current. These currents will be of equal magnitude since a floating shield collects zero net current. Due to the collection of electrons at the cathode end of the shield, and ion curent at the anode end of the shield, there will be a circulating current through the coils which make up the shield.

It is not necessary that the coil, which produces the axial magnetic field, be connected only to the stationary contact of the interrupter. In yet an alternative embodiment of the invention, the coil is electrically connected between the movable contact of the interrupter and the internally-located condensing shield.

, In a still further embodiment of the invention, a condensing shield of a fixed-gap vacuum device is con- 1 nected via a coil strap to one of the two fixed electrodes.

Further objects and advantages will readily become apparent upon reading the following specification, taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view taken through a vacuum-type circuit interrupter embodying the principles of the present invention, the contact structure being illustrated in the open-circuit position;

FIG. 1A shows a modification of FIG. 1 wherein the condensing shield is solely supported by the magnetic field coil strap;

FIG. 2 is a sectional view taken substantially along the lone IIII of FIG. 1 looking in the direction of the arrows;

FIG. 3 illustrates a modified-type circuit-interrupter construction in which the axial magnetic fieldproducing coil is disposed externally of the envelope, and is connected to the internal condensing shield by means of a probe extending through the side wall of the envelope;

FIG. 4 is another embodiment of the invention utilizing an external axial magnetic field-producing coil with the coil connected to the contact stem and also to the end plate of the circuit-interrupter, with the condensing shield likewise affixed to the aforesaid end plate;

FIG. 5 illustrates a vertical sectional view taken through a modified-type of circuit-interrupter construction in which the condensing shield assumes a spiral-strap configuration, and is electrically connected to an end plate of the circuit-interrupter;

FIG. 6 illustrates still another embodiment of the invention in which a split condensing shield of the circuitinterrupter has two coil-strap connectors electrically connected to the two separable contacts of the interrupter to result in an axial magnetic field interiorly of the vacuum bottle;

FIG. 7 illustrates still another embodiment of the invention utilizing a floating condensing shield of a generally spiral configuration in which circulating currents create the axial magnetic field within the interrupter envelope;

FIG. 8A is a diagrammatic view illustrating the principles involved in the interrupter of FIG. 7 for creating axial magnetic fields via circulating currents in a floatingcondensing shield;

FIG. 8B is, a diagrammatic view, similar to that of FIG. 8A, further illustrating the circulating currents involved in the circuit-interrupter of FIG. 7;

FIG. 9 is a graph illustrating the principles of arcvoltage reduction by means of axial magnetic fields created by ion currents to the condensing shield;

FIG. 10 is a graph illustrating the relationship between the magnetic field created by current flow through coils of different geometries;

FIG. 11 is a graph which shows how ioncurrent flow through the coils produces stable operating points, which are a function of the coil geometry;

FIG. 12 is a diagrammatic view of the test circuit, which ws used to verify that the burning voltage of high-current vacuum-arcs is significantly reduced by connecting the condensing shield via a coil to one of the electrodes. These tests were performed on the interrupter of FIG. 12A which has a geometry similar to FIG. 3;

FIG. 13 is a graph of experimental data taken from the test circuit of FIG. 12. The are voltage for a 36 Hertz a.c. arc is plotted as a function of arc current to 20 K.A. r.m.s. The are voltage is plotted first with the condensing shield connected directly to the cathode, secondly with the condensing shield connected to cathode potential via an external coil of six turns, and finally with the condensing shield connected to the cathode via a coil of 12 turns; I

FIG. 14 is one datum point taken from the curve of FIG. 13. In the graph of FIG. 14 we have shown the arc voltage during a half-cycle of arcing at a current level of 15 K.A. r.m.s. This curve was obtained for a vacuum-interrupter configuration, where the condensing shield was connected to the cathode via a six-turn field coil. We have also plotted the power expended within the interrupter;

FIG. 15 is a graph in which arc-voltage and power are again plotted for this 15 K. amp arc. Here the condensing shield is connected directly to cathode potential;

FIG. 16 is a graph which shows the peak shield-ion current plotted as a function of the 36 Hertz arc current. Three curves are shown. The first curve shows the ion current collected to the condensing shield when the latter is connected directly to the cathode. The second curve shows the ion current to the condensing shield when this shield isconnected to the cathode via a coil of six turns. Finally, we also show an experimental curve where the ion current was observed with the condensing shield connected to the cathode via 12 turns;

FIGS. 17-19 show typical oscilligrams of the ion current collected to the condensing shield during a.c. arcing at l5 I(.A. r.m.s. In FIG. 17 the peak ion current is 3.2 and the condensing shield is connected directly to the cathode. In FIG. 18 the peak ion current is l K.A., and the condensing shield is connected to the cathode via a six-turn coil. In FIG. 19, the peak ion currentis 0.42 I(.A., and the condensing shield is connected to the cathode via a l2-turn coil;

FIG. 20 is a graph showing instantaneous values of arc-voltage and arc-power during 60 Hertz arcing at 14.5 I(.A. r.m.s. This figure was determined with the condensing shield connected to cathode potential by way of a six-turn coil;

FIG. 21 is a graph which shows instantaneous values of arc-voltage and arc-power during 60 Hertz arcing at 14.5 I(.A. r.m.s. These curves were determined for a condensing shield configuration in which the condensing shield was connected directly to cathode potential; FIG. 22 is a graph which demonstrates that axial magnetic fields improve high-current interruption abil- 'ity. Although the curves in FIG. 22 were determined by applying axial magnetic fields created by field coils in series with the arc current, they serve to demonstrate the superior interruption ability. Open points are interruption points observed with no axial magnetic fields applied, and the solid points are interruption tests observed with axial magnetic field applied;

FIG. 23 is a somewhat diagrammatic view indicating the instantaneous-current conditions in which the stationary contact acts as a cathode and the moving cooperable contact acts as a anode;

FIG. 24 is a view, somewhat similar to that of FIG. 23, indicating the instantaneous current conditions at aperiod of time in which the stationary contact acts as an anode and the cooperable moving contact acts as a cathode;

FIG. 25 is a photograph of a 1,000 amp. are burning between a large-area cathode and a small-area anode. Here, there is an applied axial magnetic field;

FIG. 26 is a photograph of a vacuum arc of the same are current burning between the same electrode geometries as in FIG. 25. Here there is no axial magnetic field; and,

FIG. 27 shows an application of the invention to a triggerable fixed-gap arc device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS coils carry the series, current at all times in the closedcircuit position of the devices.

It is, however, an important feature of the present 1nvention that in the closed-circuit position of the circuit interrupter embodying applicants invention, the contacts, when closed, completely bypass the magnetic field structure, which only during the time of arcing, sets up the axial magnetic field internally of the interrupter to benefit the interruption process.

This has the important advantage that heating effects are thus eliminated in the closed-circuit position of the interrupting device, whereas the axial magnetic field structures of the prior art, by having the coils energized at all times, must put up with the heating problem, and alsowith the eddy-current heating problem, both of which are obviously undesirable.

Referring to the drawings, and more particularly to FIGS. 1 and 2 thereof, the reference numeral 1 generally designates a vacuum-type circuit interrupter comprising an evacuated envelope 2 having end plates 3 and 4. Affixed to the upper end plate 3 of the interrupter l is a stationary contact stem 5, to the lower end of which is secured the stationary contact, or electrode 6 of the circuit-interrupter 1.

Making separable cooperating engagement with the stationary electrode 6, is the movable contact or electrode 7, actuated by any suitable external operating means, not shown, and being affixed to the upper end of a flexible metallic bellows 8. The lower end 8a of the bellows 8 is secured within an opening 9 of the lower end plate 4 of the interrupter 1. As well known by those skilled in the art, the bellows 8 provides a suitable vacuum-tight seal for the operation of the movable stem 10 of the movable contact structure 7. The closed-circuit position of the device is illustrated by the dotted lines 11, whereas the full lines as shown in FIG. 1 indicate the fully open-circuit position of the device.

Disposed within the insulating casing 12, which may be of a suitable ceramic material, for example, is a metallic condensing shield 13. As well known by those skilled in the art, the condensing shield 13 insures that no metallic vapor, sputtered from the contacts 6, 7 during the arcing conditions, will be depositedinteriorly on the inner casing wall 12a, which might, if not prevented, result in internal flashover due to the voltage existing between the contacts 6 and 7 in the opencircuit position of the device 1. For example, the device may operate at a voltage of 15 K.V., and the inner surfaces 12a of the ceramic casing 12 must withstand B.I.L. levels far in excess of 15 I(.V. in the open-circuit position of the device 1.

It will be noted that the condensing shield 13 is partially supported from, and is in good electrical contact with, the upper stationary contact stem 5 by means of a coiled metallic strap 19 constituting a magnetic field coil. Thus, the condensing shield 13 is at the same voltage potential as the upper stationary contact 6. Additional support for the condensing shield 13 may be provided by an annular flange portion 15, which extends within an annular recess 17 provided on the inner wall 12a of the casing 12. The condensing shield 13 is supported by the coiled strip of metal 19, and it will be noted that, consequently, the one turn of the magnetic field coil 19 produces an axial magnetic field within the interruper 1 during arcing.

The operation of the shield coil 19 will be described first relative to the important condition, where the shield, 13 is connected to cathode potential during arcing. In the absence of a confining magentic field, the probability of arcing to the shield is high. The plasma is in good electrical contact with the biased shield, and

experiments to 20 K.A. have shown that the shield can receive an ion current i of approximately 10% of the arc current I. However, with a shield design, as shown in FIGS. 1 and 2, the shield current from the plasma must flow to the cathode stem via the coil, and will consequently produce a confining axial magentic field. The arc voltage and the probability of arcing to the shield will be reduced.

In the prior art, several investigators have disclosed that axial magnetic fields are effective in increasing the current interruption rating of vacuum interrupters. This axial magnetic field is applied parallel to the axes of the electrodes, and the field-producing coils are always placed in series with the current through the electrodes. In patents by Lee and Porter, the axial magnetic field is produced by a solenoidal coil wound external to the interrupter. In a third patent by Porter, the electrode stems are constructed from materials of widelydifferent electrical conductivities, and an axial magnetic field is produced from these stem coils.

We find that the axial magnetic field is effective in increasing the current-interruption ability of a vacuum interrupter for two reasons. First, the arc voltage is reduced during the critical half-cycle of arcing prior to the natural current zero. This arc-voltage reduction minimizes the energy released within the interrupter, and consequently minimizes the temperature rise of the internal interrupter components. In particular, recent experiments indicate that axial fields produce a significant increase in the threshold current associated with anode-spot formation. Thus, in low-current d.c. experiments, where a small-diameter anode is drawn to long electrode spacings from the cathode, the threshold current for anode-spot formation in the absence of an axial magnetic field is 500 A. When the experiment is repeated with an axial field of 300 G, however, the threshold current for anode-spot formation is raised to 1,500 A. Recent literature reveals that low-current anode spot experiments are relevant to high-current phenomena in vacuum interrupters, and we are therefore confident that axial magnetic fields will delay the onset of anode-spot formation in a practical a.c. interrupter. Comparative arc structures observed at 1,100 A. with and without an axial magnetic field are shown in the photographs of FIGS. 25 and 26.

A second benefit derives from the confining effect of the axial magnetic field. This field reduces the plasma contact with the vapor condensation shield, and therefore reduces the probability of the are striking to the shield. This is a particularly important effect since costreduced vacuum interrupters are being manufactured with the vapor-condensation shield connected directly to one of the electrodes. We have shown that the rate of rise of dielectric strength is only decreased slightly in these integral shield designs, but problems may be encountered at high currents (approximately K.A.) due to the tendency for the arc to strike over to the shield. The strikeover tendency will be small when the shield is connected to the anode electrode during arcing. However, the shield will be biased negative relative to the arc plasma during those arcing half-cycles in which the shield is at cathode potential. Under these circumstances, the arc plasma may initiate cathode spots on the shield with resulting catastrophic failure of the interrupter. We can conclude that axial magnetic fields will definitely prove of value in cost-reduced interrupters, and the field will prove of particular value when the shield is at cathode potential during the arcing half-cycle.

The operation of the shield coil will be described first relative to the important condition where the shield of FIG. 1 is connected to cathode potential during arcing. In the absence of a confining magnetic field, the probability of arcing to the shield is high, as shown in FIG. 23. The plasma is in good electrical contact with the biased shield, and our experiments have shown that the shield can receive an ion current i of approximately 10% of the arc current I. However, with a shield design, as shown in FIGS. 1 and 23, the shield current from the plasma must flow to the cathode stel via the coil 19, and will consequently produce a confining axial magnetic field. The are voltage and probability of arcing to the shield are reduced. It must be appreciated that the coil 19 of FIG. 23, connecting the shield to the electrode stem, may be placed external to the interrupter, rather than internal, as shown in FIG. 3. Under these circumstances the shield is electrically isolated from both electrodes internally, but is biased to one of the electrodes via the coil and an electrical connection to the shield-support flange.

Concentrating our attention on the shield design of FIG. 1, let us consider the possible magnitudes of the magnetic field produced by the metallic coil 19. If we approximate the metallic coil 19 of FIGS. 1 and 2 by a single loop of radius R equal to the shield radius, then the magnetic field in the interelectrode gap is given by the expression:

us-I'M T (I) where x is the distance from the coil plane to a point in the interelectrode gap. Since the coil can be located in the electrode plane, let us neglect x-compared to R.

Then equation (1) reduces to the form:

1 al-in! (amperes per meter).

But

where 11,, is the permeability of free space (4 11' X l07 henry metre Whence F (gauss) x 10 In present interrupter designs R has a value of 3 to 4 cm. Allowing i, the ion current, to be approximately 10% I, the arc current; then:

E=I 2 X 10 gauss ample, is of the order of 38 V. at 4.2 K.A. in the absence of a magnetic field. When a field of 200 G. is applied via external Helmholtz coils, the arc voltage reduces to 30 V., and the ion current i still has a value of 220 A. Thision current couldha ve produced the required magnetic field by flowing through four turns of the described loop.

An alternative shield design is shown in FIG. 5. Here the shield is formed from a spiral strip of metal, and these strips overlap in order to protect the insulating envelope from vapor deposition. In FIG. the shield is shown connnected to the stationary electrode, although either electrode support would prove satisfactory.

Let us now consider the less important situation of the shield connected to the arcing anode, as shown in FIG. 24. This connection is less important since the shield is now biased positive with respect to the plasma, and cathode spots are unable to form on the shield surface. Thus, the probability of the are striking to the shield is low, even without a confining magnetic field. In the absence of a field, a shield connected to the anode acts as an auxiliary anode and draws a net electron current from the plasma. This is beneficial since the threshold current for anode-spot formation increases with the effective anode area. With coils connected to the shield, as shown in FIGS. 1 to 5, this electron current will be decreased to some extent by the resulting axial magnetic field. However, we can expect that the resulting decrease in the anode-spot threshold current will be offset by the confining effect of the magnetic field. In particular, we have observed experimentally that the arc voltage of high-current arcs to 25 K.A. is significantly reduced when the shield is connected to anode potential via approximately six turns of coil.

If necessary, a split-shield device can be designed, in which one-half of the shield is connected via a coil to each of the electrodes. A schematic of such a device appears in FIG. 6. Here the shield is split by a cut parallel to the axes of the electrodes. However, the shields could be split by a cut parallel to the electrode surfaces.

A final concept concerns the application of a shieldgenerated axial magnetic field during arcing, even when the shield is electrically insulated from both electrodes. Let us suppose that the floating shield of FIG. 7 has a generally spiral configuration adjacent to the arcing gap.

Since the floating shield is isolated from both electrodes, the shield, during arcing, collects zero net current from the adjacent arc plasma. However, we can confidently expect that the shield will collect electron current at regions adjacent to the cathode electrode, and an equal ion current to regions adjacent to the anode electrode. As a consequence of this distribution of current collection, a circulating current will pass through the spirals of the shield, and will produce an axial magnetic field. We will clarify the mechanism of FIG. 7 by referring toFIGS. 8A and 8B. In FIG. 8A we show an arc of current I burning between an anode and a cathode. We are also showing that the arc is burning through the hole bored in two metallic plates, plate 1 and plate 2. Let us consider that the plates are isolated from each other, and that there is an appreciable voltage drop in the arc column. Let us consider that the total are voltage is perhaps 150 volts, and that the plasma potential adjacent to plate 1 is approximately 100 volts positive relative to cathode potential, and

that theplasma potential in the arc column adjacent to plate 2 is 50 volts positive relative to cathode potential. From Langmuir probe considerations, we know that the potential of plate 1 will be approximately volts positive relative to the cathode, and the isolated potential of plate 2 will be approximately 50 volts positive relative to cathode potential, that is, the plates assume the potential of the adjacent plasma. Now consider the effect of connecting plates 1 and 2 via an ammeter. This is shown by the dashed lines in FIG. 8A. The potential of plate 1 will drop from 100 volts positive relative to cathode to approximately the potential of plate 2, that is, the potential of plate 1 will drop from I00 volts to 50 volts. As a consequence, plate 1 is effectively biased minus 50 volts negative relative to the potential of the adjacent arc plasma. It will consequently draw a net ion current I, from the arc plasma. Due to the high electron mobility, plate 2 will rise only several volts positive relative to the adjacent arc plasma, that is, it might rise from initially 50 volts positive relative to cathode to 52 or 53 volts positive relative to cathode. Thus, plate 2 is effectively biased positive relative to the adjacent arc plasma, and will draw a net electron current 1 from the arc plasma. Thus, the two metallic plates 1 and 2 draw zero net current from the arc plasma, but the ammeter connecting the two plates together will register a total circulating current I,. It is this circulating current which we will use to generate our axial magnetic field.

In FIG. 88, we are depicting that the two ends of the metallic shield, which we can relate to plate 1 and plate 2 of FIG. 8A, are connected together via a strip of metal in the shape of a coil. Circulating currents through this coil will generate the desired axial magnetic field.

In summary, the shield configuration of FIG. 7 will create an axial magnetic field due to the fact that there will be a significant voltage drop along the high-current arc-column adjacent to a shield of considerable overall length. This voltage drop will drive a circulating current through the shield coils.

In clarifying the mechanism whereby shield currents create axial magnetic fields as, for example, in FIG. 1, we will concentrate on describing the situation of shield biased to cathode potential. This connection is of particular importance. When a shield is connected to anode potential, even without shield coils, the shield acts like an auxiliary anode, and markedly reduces the arc voltage. However, if a shield is connected directly to cathode potential there is no reduction in the arc voltage. A shield connected via coils to either electrode does produce a reduction in the arc voltage, and we will now explain, in detail, the mechanism of voltage reduction with shields connected to the cathode. Let us first consider FIG. 10. If we take any coil of a given geometry, the magnetic field strength at the center of the coil will increase linearly with the current through the coil. For a given current in the coil, the field strength will be larger, the greater the number of turns in the coil. For the various embodiments of our invention, the coil is, of course, the metal coil whether external or internal connecting the shield to one of the electrodes. This current is an ion current when the shield is biased to the cathode. In FIG. 10, it is shown that as the ion current through a given coil increases, the axial magnetic field will also increase. This ion current is drawn to the shield from the arc plasma. Naturally, an increasing axial magnetic field, generated by thecoil, will decrease the arc plasma contact with the shield. Consequently, there are limited operating points determined by the ion current dependence on the axial magnetic field and the shield geometry.

Consider FIG. 11. At a given arc current, and no magnetic field, a large ion current will be collected to a negatively-biased shield. The intercept on the Y-axis shows the ion current to the shield when the shield contains no turns. From experiment we know that the ion current to a shield with on turns decreases with an externally-applied axial magnetic field. This decreasing ion current is shown by the dashed line of FIG. 11. Consider now a shield with a number of turns between shield and cathode-By analogy with FIG. 10, the created axial magnetic field increases linearly with the ion current to theshield. For a given are current, the shield will operate at the stable operating point given by the intersection of the shield load-line and the ion current curve. Consider further the practical experimental data of FIG. 9- This figure was generated from data using a practical vacuum interrupter with an externally applied axialnia'gnetic field. The data all refers to an arc current of 4.2 K.A. Let us first consider the arc-voltage curve. When there is zero axial magnetic field the arc voltageis 38 volts. With an applied magnetic field of slightly less than 200 gauss the arc voltage has reduced to 30volts. The arc voltage then increases slowly with further increase in magnetic field. Consider now the ion current curve. This curve was determined with the shield connected directly to cathode potential, but for various values of externally-applied axial magnetic field. In the absence of an axial field, the ion current from the 4.2 K.A. arc plasma has a value of about 280 amps. As the shield applied to the arc plasma increases, this ion current reduces and reaches a value of about I20 amps when the field is 800 gauss.

Now, consider the arcing situation where, instead of applying external axial magnetic fields, the shield is connected to cathode potential via several turns. By analogy with FIGS. 10 and 11, the shield will have a coil-load line. This coil-load line is shown in FIG. 9 and intersects the ion current curve at point A. Let us consider the possibilities. It will be obvious that the arc will not burn with zero magnetic field. For that particular hypothetical situation, an ion current of 280 amperes would flow from the arc plasma to the shield and the arc voltage would be 40 volts. However, a shield ion current of 280 amps would create a large magnetic field dictated by the coil-load line. Thus, an operating point at the zero magnetic field is unstable. The only stable operating point is point A. It will be noted in FIG. 9 that the arc-voltage minimum is attained at relatively low magnetic fields. Fortunately, these magnetic fields can be created using coils of only several turns.

As direct evidence of the shield-coils efficiency in reducing the arc voltage, let us consider the experimental data of FIGS. 13, 16, 17, 18 and 19. We experimented with a vacuum interrupter, as shown in FIG. 12A. Here the shield coil is connected externally to the cathode 'via a six-turn coil. This is similar to the embodiment I to the shield. FIG. 13 shows the peak arc voltage during a half cycle of arcing at currents to 20 K.A. The are voltage at 15 K.A., for example, with the shield connected directly to the cathode is approximately 150 volts. With the shield connected to cathode potential, via a six-turn coil, the arc voltage is approximately volts. With the shield connected to cathode potential.

via a twelve-turn coil, the arc voltage is 55 volts. The curve of FIG. 16 shows the current which flows to the shield from the arc plasma for the experimental conditions of FIG. 13. At l5 I(.A. an ion current of approximately 3 K.A. flows to the shield when the latter is connected directly to the cathode. This connection corresponds to the zero-field situation of FIG. 9. With the shield connected to the cathode via six turns, the peak ion current, during the arcing half cycle, reduces to approximately 1 kiloampere. Thus, in this situation, the operating point A of FIG. 9 has resulted in a one-third reduction in the ion current. However an ion current of l kiloampere flowing through the six-turns produces a marked effect on the arc voltge, as already discussed in FIG. 13. Again, with reference to FIG. 16, with the shield connected to cathode via 12 turns, the ion current through the coil turns is reduced to approximately 0.6 K.A. Typical data for FIG. 16, obtained using the configurations of FIG. 12A, appear in FIGS. l7, l8 and 19. These figures are taken from single oscillograms of shield currents during a half cycle of arcing at 15 K.A. R.M.S. FIG. 17 shows the ion current to the shield when the latter is connected directly to cathode potential. This is the high arcing-voltage condition.

FIG. 18 shows that the ion current has been markedly reduced by connecting the shield via six turns to cathode potential. The arc voltage is now significantly reduced. FIG. 19 shows that the ion current is even more reduced when the shield is connected to cathode potential via 12 turns. Thisis the lowest voltlage condition. Again, the reasons for the ion current reductions observed in FIGS. 17, 18 and 19, for a given 15 K.A. arc but with different shield geometries, can be understood relative to FIG 1 l. The test data, which we have discussed, were obtained in the interrupter of FIG. 12A

using the test circuit shown in FIG. 12. This test circuit comprises a two megajoule capacitor bank which discharges through a reactor. In order to obtain arcvoltage data at comparative electrode separations, we deliberately obtained our data using a circuit tuned to 36 cycles per second. However, we obtained similar data to FIGS. 13 and 16 when we performed experiments at 60 cycles per second. Examples of data at 60 cycles appear in FIGS. 20 and 21. Data in these two curves was obtained during a half cycle of arcing for an arc current of 14.5 K.A. R.M.S. Consider FIG. 21. The shield is connected directly to cathode potential, and the arc voltage during the 8 milliseconds of arcing is high. The peak arc voltage during the arcing half cycle is approximately volts. In FIG. 21 we have also plotted the instantaneous power dissipated in the interrupter. From this figure we estimate that an are energy of 8.9 kilojoules was expended during the arcing half cycle. Consider now FIG. 20. Here the peak arc voltage is markedly reduced by using a six-turn coil between the shield and the cathode. In particular, the arc energy, expended during the off arcing half cycle, is only 4.9 kilojoules. Thus, the effect of the shield coils is to reduce not only the peak arc voltage, but also, and more importantly, the total energy expended in the interrupter during the arcing half cycle. Since the data of 

1. A circuit-interrupting device of the vacuum type comprising, in combination: a. means defining a highly-evacuated envelope including an insulating casing; b. a pair of electrodes disposed within said highly-evacuated envelope and having a fixed gap therebetween at least in the fully-open position of said interrupting device; c. means for at times establishing an arc across said gap between the two electrodes; d. metallic condensing-shielding means disposed interiorly of said highly-evacuated envelope for preventing deposition of metallic vapor from the arcing region onto the inner walls of said insulating casing for preventing flashover across the insulating casing; e. magnetic field-producing means for developing across said gap an axial magnetic field that has its lines of force extending across said gap generally parallel to the longitudinal axis of said arc; and, f. said magnetic field-producing means including a field-coil electrically interconnecting said metallic condensing shielding means to one of said electrodes.
 2. The circuit-interrupting device of claim 1, wherein the pair of electrodes are constituted by a pair of separable contacts which make engagement and disengagement with each other.
 3. The combination according to claim 1, wherein the metallic condensing-shielding means is generally cylindrical and the field coil is disposed within the cylindrically-shaped metallic condensing shield.
 4. The combination according to claim 2, wherein the metallic condensing-shielding means is generally cylindrical and the magnetic field coil is disposed interiorly of the cylindrically-shaped metallic condensing shield.
 5. The combination according to claim 2, wherein the field coil electrically interconnects the condensing shielding means to the stationary contact of the device.
 6. The combination according to claim 2, wherein the field coil electrically interconnects the condensing shielding means with the movable contact of the interrupting device.
 7. A vacuum-type alternating-current circuit interrupter including means defining an evacuated envelope having an insulating casing, separable contacts disposed within said evacuated envelope and separable from each other to establishing arcing between the separated contacts, condensing-shield means for preventing the deposition of metallic vapor from the region of arcing onto the inner wall of the insulating casing, and a conducting strap electrically interconnecting the condensing shield means with one of the separable contacts, whereby a magnetic field coil is thus provided to produce an axial magnetic field within the evacuated envelope and in the region of the arcing gap.
 8. The combination according to claim 7, wherein the conducting strap is electrically connected to the stationary contact of the interrupter.
 9. The combination according to claim 8, wherein the conducting strap is electrically connected to the stationary contact stem.
 10. The combination according to claim 7, wherein a field-coil interconnects the condensing shield to one of the contacts and said field coil is disposed externally of the evacuated envelope.
 11. The combination according to claim 10, wherein the support for the condensing shield constitutes a conductor electrically interconnecting the condensing shield with the externally-disposed field-coil.
 12. The combination according to claim 7, wherein the evacuated envelope has a metallic end-plate and an insulating wall portion separates the end plate of the device and the stationary contact stem, and an externally-disposed field-coil surrounds said insulating portion and is electrically connected between the stationary contact stem and said end plate, and said condensing shield for the interrupter is electrically connected to said one end plate.
 13. A vacuum-type circuit interrupter comprising means defining an evacuated envelope having a metallic end plate, a pair of separable contacts disposed interIorly within said evacuated envelope and separable to establish arcing, a condensing-shield means comprising a spiral strip of metal with the strips overlapping in order to protect the inner wall of the evacuated envelope from vapor deposition, and said spiral strap of metal being electrically connected to said one end plate of the interrupter.
 14. The combination according to claim 13, wherein said one end plate supports the stationary contact stem of the device.
 15. The combination according to claim 13, wherein the spiral of the condensing-shield means are of increasing diameter in a direction away from said one end plate.
 16. The combination according to claim 13, wherein said spiral strip is supported from said one end plate and is in good electrical contact therewith.
 17. The combination according to claim 7, wherein the condensing shield is split and is electrically connected by two straps to both of the separable contacts.
 18. The combination according to claim 7, wherein the condensing shield is electrically floating and comprises a pair of end loops with a coil electrically interconnecting the two end loops. 